1. Field of the Invention
The present invention relates to a centrifugal liquid separation machine and in particular to a screw type centrifugal liquid separation machine that lifts grit and other solids from the bowl wall in a radially inward manner and resuspends the grit and other solids into the heavy phase discharge flow.
2. Description of the Related Art
Centrifugal machines are useful in many types of applications. In one application, wastewater treatment plants, it is desired to achieve a 4% to 6% cake solids discharge. This range of cake solids is required in order for an anaerobic digester to operate efficiently. Falling below this range requires increased digester capacity. Rising above this range typically results in mixing problems due to the thickness of the heavy phase liquids. Even though the principles of the present invention are described with respect to one type of application, it is understood that the invention is in no way limited to this described application.
In the basic form, decanter type centrifugal separation machines have a rotating outer bowl, an internal screw conveyor co-axially aligned with the outer bowl, and a mechanism for maintaining a difference in speed between the rotating outer bowl and the internal screw conveyor to allow for continuous operation of the machine. Rotation of the bowl at elevated speeds results in solid liquid separation action within the separation region of the machine due to elevated levels of gravitational forces within the machine. Materials such as solids and heavier density liquid will thus settle to the outer diameter of the separation region and the lower density liquid will migrate to the inner diameter of the separation region. The separation rate increases with the elevation of gravitational forces resulting from the rotation of the bowl. The screw conveyor has a rotational speed greater or less than the rotational speed of the outer bowl. This difference in speed allows screw conveyor flights to provide a mechanical sweeping action within the separation region. Grit tends not to flow, and as such, needs to be conveyed to discharge from a decanter centrifuge. However, in a design where the solids discharge is radial and inward, this can create an accumulation problem. The accumulation of non-flowing materials can increase operational horsepower, reduce capacity and otherwise disrupt proper operation of the device. The grit also tends to grind upon parts and subject those parts to premature failure. Hence, a design adequately dealing with grit would be advantageous.
There have been many centrifuge designs over the years. A few of those designs are illustrated in the following US patents in order to illustrate to current state of centrifuge devices.
United States Patent Number (hereafter “USPN”) U.S. Pat. No. 3,795,361 to Lee is titled Centrifuge Apparatus. This patent describes how a decanter centrifuge having a screw conveyor within an imperforate bowl is provided with an annular baffle carried by the screw conveyor. A heavy phase discharge port is taught to be located in a tapered portion of the bowl and is located at a greater radial distance from the rotational axis than the inner surface of the light phase material. The periphery of the baffle is closely spaced from the bowl in order to form a restricted passageway for the underflow of heavy phase material from a separating zone within the cylindrical portion of the bowl to a heavy phase discharge zone within the tapered portion of the bowl. With a conical baffle, incoming feed is directed onto the inwardly facing surface of the baffle and accelerated in order to minimize turbulence in the separating zone. The use of a tapered portion, or a beach, reduces the capacity of the machine, as shallow beach angles required to adequately convey grit or trash requires an undesirably large proportion of bowl length.
U.S. Pat. No. 4,339,072 to Hiller is titled Centrifuge for Separating Solids/Liquids Mixtures. In this invention, a centrifuge drum having an outer jacket is provided with apertures positioned in the jacket. Through the apertures at least a partial discharge of concentrated solids phase occurs thereto. A control device preferably in the form of a disk provides a surface spaced at a small interval from the apertures so as to prevent the flow of solids/liquids through the aperture except when a discontinuity such as a recess or cut-out in the surface occurs so as to allow flow through the aperture. While this patent describes a solution for eliminating a truncated cone by discharging from the outer bowl, its design is not without drawbacks. For example, it is required that all solids pass through very small nozzles. This can result in undesirable amounts of abrasive damage and plugging of the machine, especially when grit is present.
U.S. Pat. No. 5,542,903 to Nishida et al. is titled Centrifugal Liquid Separating Machine Using Deceleration Vanes. This patent teaches that discharge passages for concentrated and separated liquids are separately formed in shafts of a rotary bowl and a screw conveyor. In an inlet passage of the radial discharge passage leading from the inside of the rotary bowl to the discharge passage in the shaft, an annular space is divided into sectors by a plurality of deceleration vanes which are mounted on the screw conveyor and extend in a radial direction from the axis of the machine. While this patent shows a solution to problems with amorphous trash, is does not address the problems caused by abrasive materials such as grit.
U.S. Pat. No. 4,449,967 to Caldwell is titled Conveyor Flight Configuration. This patent shows that the blade of a helical screw conveyor of centrifuge apparatus can be improved to reduce the amount of torque required in operating the apparatus as well as to produce discharged solids drier than the solids discharged using a conventional blade. The patent teaches the improvement to be that the distal end of the leading surface of the blade at or adjacent at least the trunco-conical end of the centrifuge bowl have a generally arcuate configuration concave in the direction of the solids discharge port such that the more distal blade portions peel the separated solids material from the bowl wall with minimum torque requirements, while the less distal arcuate portions tumble the solids to reduce the moisture content. Typically, the blade's arcuate leading surface is formed by add-on, wear-resistant members attached to a backing plate which in turn is mounted on the leading surface of the blade.
U.S. Pat. No. 5,653,673 to Desai et al. is titled Wash Conduit Configuration in a Centrifuge Apparatus and Uses Thereof. This patent shows a centrifugal apparatus for the continuous separation of solids-liquid mixtures and the internal washing of separated solids with one or more conduits to deliver washing liquid into centrifugally sedimented solids tumbling, which allows wash liquid to better penetrate a less compacted pile of solids. A distal end of each wash conduit is positioned in close proximity to the inner surface of the centrifuge bowl near the conveyor blade which is adapted to contact the solids upon separation and propel separated solids toward the solids discharge port. The invention includes centrifugal apparatus provided with plurality of cutting tools such as knife blades adapted to cut and separate the sedimented solids. In another aspect of the invention centrifugal apparatus is provided with one or more dip weirs dividing the centrifugally separated liquid pool in the bowl into axially adjacent zones with suitable a passageway for transfer of liquids and solids between adjacent zones. In another aspect of the invention at least a portion of the leading surface of the conveyor blade is provided with an arcuate surface portion adjacent to the distal edge thereof of defined concavity in the leading surface and shape adapted to contact the sedimented solids and tumbling them in washing liquid. This invention provides improved separation of mother liquor from discharged solids and/or reduction of washing liquid required to achieve desired purity of recovered solid product.
None of these patents teach a way to radially and inwardly lift grit and other solids from the bowl wall.
None of these patents teach a way to resuspend grit and other solids in a heavy phase discharge stream.
None of these patents illustrate wipers used to prevent plugging of a solids baffle.
Thus, there exists a need for a centrifugal liquid separation machine that solves these and other problems.